What happened
On 16 August 2015, a RL7A XP Sherwood Ranger, registration G-CHHD, was performing a private takeoff from the grass runway at Drurode Bay Airfield, Northumberland. Approximately eight seconds into the takeoff roll, as the aircraft was approaching liftoff, the right main wheel suffered a structural failure. The broken components of the wheel dug into the turf, causing the aircraft to nose over and come to rest in an inverted position.
The pilot, who sustained minor injuries, managed to unbuckle his harness and exit the aircraft. Although several people were present at the airfield, the pilot chose to evacuate immediately due to the smell of escaping fuel. The aircraft sustained damage to the engine cowlings, wings, and propeller, and the engine experienced a shock-load.
The investigation
The investigation focused on the construction of the right wheel, which consisted of two aluminium halves secured by bolts. Components were examined by the Light Aircraft Association (LAA). The examination revealed that the outer half of the wheel had failed circumferentially, causing the outer rim to separate from the assembly.
Technical analysis identified that two of the six attachment bolts had not been fully tightened, leaving a gap of approximately 1.5 mm, while a third bolt was also found to be loose. The presence of corrosion products near one of the loose bolts indicated fretting caused by movement between the two halves during operation. Furthermore, a radial crack was identified, which was determined to be a low-cycle fatigue crack.
Findings
- The primary cause of the wheel failure was insufficient torque applied to the attachment nuts during the initial assembly of the wheel.
- The lack of proper bolt tension allowed for relative movement between the two aluminium halves, leading to fretting and the development of a low-cycle fatigue crack.
- The final separation of the wheel was caused by an overload following the fatigue failure.